A. Field of the Invention
This invention relates to the field of material forming, particularly to material forming under superplastic conditions.
B. Description of the Prior Art
Forming methods which are normally used to capitalize on superplasticity in selected materials involve the use of fluid (preferably gas) pressure to cause sheet material deformation and a configurational die into which the part is formed. Patents have been issued which relate to this process, e.g., U.S. Pat. No. 3,340,101 and 3,934,441, and the process is being used increasingly for forming such parts as titanium sheet metal structures for aircraft. In addition to the forming of a single sheet of a superplastic material, other recent processes combine diffusion bonding with superplastic forming to produce complex structures, such as sandwich structures, e.g., U.S. Pat. No. 3,927,817, and reinforced structures, e.g., U.S. Pat. No. 3,920,175.
These patents normally employ fluid pressure forming and rely upon the superplastic properties of the material to achieve high tensile elongations and controlled thinning. Since the elongation and thinning characteristics of the material being formed are related to the rate of straining, the rate of pressure application is critical to the successful fabrication of parts. Prior to this invention, the rate of pressure application was established by a trial-and-error method, resulting in much longer forming times than is possible by controlled strain-rate forming and uncertainty that suitable conditions for superplastic behavior were being sustained. Typically, the prior art method was conducted by slowly increasing the pressure applied to the metal blank to a maximum value which was then sustained to complete the forming. Both the rate of pressure increase and the maximum pressure was determined by trial and error. The goal of the empirically developed pressurization cycles was the forming of the metal blank without rupture, i.e., within the range of strain rates where superplastic behavior is pronounced.
Accordingly, prior art methods required time consuming and expensive trial and error testing and were still not capable of utilizing optimum or assuring necessary forming rates during the entire forming process, resulting in excessively long forming times. Attempts to increase the forming rate resulted in rupturing the part because the strain rate required for superplastic forming was exceeded during critical times in the forming operation. These problems have severely limited the widespread application of this promising method of forming utilizing superplastic properties of some metals and other materials.